In a wireless communication system for mobile phones or the like, base station apparatuses (eNBs; eNodeBs) configuring cells (communication service areas) for providing wireless communication services for a plurality of mobile station apparatuses (terminals or UE (user equipment)) are installed in a city and the suburbs. In particular, in the wireless communication system, a cellular configuration, in which a plurality of base station apparatuses are arranged, is used in order to expand communication areas.
In the cellular configuration, the same frequency is reused in the cells of the base station apparatuses in order to improve spectral efficiency. When inter-cell interference is caused due to the reuse of the same frequency in the cellular configuration, however, improvement of the spectral efficiency is limited.
As a method for suppressing and reducing inter-cell interference in an uplink of the cellular configuration, inter-cell interference coordination (ICIC) that uses an indicator OI (overload indicator), an indicator HII (high interference indicator), and the like is used (NPL 1). The indicator OI is a control signal used by a base station apparatus for notifying another base station apparatus that an interference level of a mobile station apparatus connected to the other base station apparatus is high when the interference level of the mobile station apparatus is high. In addition, the indicator HII is a control signal used by a base station apparatus that is receiving signals from a mobile station apparatus which is located at a cell edge of the base station apparatus and which performs transmission with high transmission power for notifying another base station apparatus that the base station apparatus is receiving signals from the mobile station apparatus.
FIG. 32 illustrates an overview of an existing wireless communication system A1000 in an uplink in which the inter-cell interference coordination ICIC is applied. A base station apparatus A1000-1 and a base station apparatus A1000-2 include a cell A1000-1a and a cell A1000-2a, respectively, and the base station apparatuses are arranged using one-cell frequency reuse so that the cell A1000-1a of the base station apparatus A1000-1 and the cell A1000-2a of the base station apparatus A1000-2 partially overlap. A plurality of mobile station apparatuses are included in each cell, and each mobile station apparatus is controlled in such a way as to be wirelessly connected to a base station apparatus that can receive signals with optimum reception field intensity.
The base station apparatus A1000-1 is connected (r11) to a mobile station apparatus A2000-1. In addition, the base station apparatus A1000-1 is interfered (r21) by a mobile station apparatus A2000-2 connected (r22) to the base station apparatus A1000-2.
The interfered (r21) base station apparatus A1000-1 transmits the indicator 10 to the base station apparatus A1000-2 through a backhaul line A10 (for example, an optical fiber, an X2 interference, or the like). Upon receiving the indicator IO, the base station apparatus A1000-2 causes the mobile station apparatus A2000-2 to stop the transmission in order to suppress and reduce the inter-cell interference.
In addition, before the mobile station apparatus A2000-2 transmits signals (r22), the base station apparatus A1000-2 transmits the indicator HII to the base station apparatus A1000-1 through the backhaul line A10. Upon receiving the indicator HII, the base station apparatus A1000-1 performs scheduling such that signals (r11) from the mobile station apparatus A2000-1 are not interfered, in order to suppress and reduce the interference.
In addition, a lot of reception dead zones and weak-field zones are being caused in these years because of high-rise buildings and apartments built in the process of rapid urbanization. In these zones, connections between mobile station apparatuses and base station apparatuses are often restricted. In addition, improvement of throughput to mobile stations is required in order to increase communication speed in a mobile communication system. Similarly, it is desired to realize high-speed communication with mobile station apparatuses located at cell edges (peripheral zones of communication service areas) without trouble.
As a method for improving throughput, a method has been proposed in which a plurality of base station apparatuses are arranged such that part or the entirety of the range of a macrocell configured by a main base station apparatus (macro base station) and the range of a cell of a low-power base station (picocell base station or femtocell base station) whose maximum transmission power is lower than that of the macro base station overlap (heterogeneous network; NPL 2).
FIG. 33 illustrates an overview of a wireless communication system 1000 in a downlink in which a plurality of base station apparatuses having different cell radii are arranged. The base station apparatuses are arranged using one-cell frequency reuse so that a cell 1000-1a (macrocell) of a main base station apparatus 1000-1 (macro base station apparatus), a cell 1000-2a (picocell) of a base station apparatus 1000-2, which is a low-power base station whose maximum transmission power is lower than that of the macro base station apparatus, and a cell 1000-3a (picocell) of a base station apparatus 1000-3 overlap. A plurality of mobile station apparatuses are included in each cell, and each mobile station apparatus is controlled in such a way as to be wirelessly connected to a base station apparatus that can receive signals with highest reception field intensity. In FIG. 33, a mobile station apparatus 2000-1 is wirelessly connected (r11) to the base station apparatus 1000-1, a mobile station apparatus 2000-2 is wirelessly connected (r22) to the base station apparatus 1000-2, and a mobile station apparatus 2000-3 is wirelessly connected (r33) to the base station apparatus 1000-3.
By constructing such a heterogeneous network, it becomes possible to improve total spectral efficiency in an area covered by the macrocell viewed from the perspective of the network.
In addition, as a method for suppressing and reducing inter-cell interference in a downlink of a heterogeneous network, a method has been disclosed in which a plurality of base station apparatuses transmit and communicate signals to mobile station apparatuses in a coordinated manner (NPL 3).
FIG. 34 illustrates a transmission frame format in the downlink of the heterogeneous network. In an upper part of FIG. 34, one frame is configured by ten subframes of a plurality of types including normal subframes and resource mapping restriction subframes (also referred to as restriction subframes). In the upper part of FIG. 34, a subframe index #1, a subframe index #3, a subframe index #4, a subframe index #5, and a subframe index #9 are normal subframes, and a subframe index #0, a subframe index #2, a subframe index #6, a subframe index #7, and a subframe index #8 are resource mapping restriction subframes. The resource mapping restriction subframes may be ABSs (almost blank subframes), MBSFNs (multicast-broadcast over single-frequency networks), or the like.
The normal subframes refer to subframes in which a base station apparatus can perform resource mapping on information data, control data, and reference signals. For example, as downlink signals in LTE, physical downlink shared channels (PDSCHs; channels that mainly transmit information data), physical downlink control channels (PDCCHs; indicated by horizontally hatched portions in the figure), synchronization signals (PSSs; primary synchronization signals and SSSS; secondary synchronization signals), physical broadcast channels (PBCHs), cell-specific reference signals (CRSs), and the like may be subjected to the resource mapping.
The resource mapping restriction subframes are subframes in which a base station apparatus is restricted to resource mapping of certain signals. In an ABS, only CRSs and/or certain control signals (SSSs, PSSs, PBCHs (checkered portions in the figure), and the like) are arranged (the subframe index #0 in the upper part of FIG. 34). In an MBSFN subframe, only CRSs are arranged (the subframe index #2, the subframe index #6, the subframe index #7, and the subframe index #8 in the upper part of FIG. 34). In ABS and MBSFN subframes, signals (for example, PDSCHs) other than the above-mentioned signals that can be arranged are not arranged (hatched portions in the figure).
A lower part of FIG. 34 illustrates a downlink transmission frame format at a time when the base station apparatus 1000-2 and the base station apparatus 1000-3 transmit signals to their respective connected mobile station apparatuses. In the lower part of FIG. 34, one frame is configured by ten normal subframes. In FIG. 34, information data (PDSCHs) transmitted from the base station apparatus 1000-1 to the mobile station apparatus 2000-1 is arranged in the subframes other than the subframe index #0, the subframe index #2, the subframe index #6, the subframe index #7, and the subframe index #8 in the upper part of FIG. 34. Information data transmitted from the base station apparatus 1000-2 to the mobile station apparatus 2000-2 is arranged in a subframe index #0, a subframe index #4, a subframe index #5, a subframe index #6, and a subframe index #8 in the lower part of FIG. 34. Information data transmitted from the base station apparatus 1000-3 to the mobile station apparatus 2000-3 is arranged in the subframe index #0, the subframe index #4, the subframe index #5, the subframe index #6, and the subframe index #8 in the lower part of FIG. 34.
Thus, since the base station apparatus 1000-2 and the base station apparatus 1000-3 assign the information data for the mobile station apparatus 2000-2 and the mobile station apparatus 200-4, respectively, that can be interfered by the base station apparatus 1000-1 in the subframes synchronized with the subframes in which the base station apparatus 1000-1 does not arrange information data, inter-cell interference from the base station apparatus 1000-1 can be reduced.